Buggy code:
public abstract class PoiSheet < WORKBOOK extends Workbook , SHEET extends Sheet , ROW extends Row , CELL extends Cell , CELLSTYLE extends CellStyle , COLOR extends Color > implements Sheet { protected final SHEET poiSheet ; private int firstColumn ; private int lastColumn ; private List < RectangularRegion > mergedRegions ; public PoiSheet ( SHEET poiSheet ) { } public SHEET getPoiSheet ( ) { } public RectangularRegion getMergedRegion ( int rowNum , int colNum ) { } private void update ( ) { firstColumn = Integer . MAX_VALUE ; lastColumn = 0 ; <START_BUG> for ( int i = poiSheet . getFirstRowNum ( ) ; i < ( poiSheet . getLastRowNum ( ) ) ; i ++ ) { <END_BUG> final org . apache . poi . ss . usermodel . Row poiRow = poiSheet . getRow ( i ) ; if ( poiRow != null ) { final short firstCellNum = poiRow . getFirstCellNum ( ) ; if ( firstCellNum >= 0 ) { firstColumn = Math . min ( firstColumn , firstCellNum ) ; } final short lastCellNum = ( ( short ) ( ( poiRow . getLastCellNum ( ) ) - 1 ) ) ; if ( lastCellNum >= 0 ) { lastColumn = Math . max ( lastColumn , lastCellNum ) ; } } } if ( ( firstColumn ) == ( Integer . MAX_VALUE ) ) { firstColumn = 0 ; lastColumn = 0 ; } final int numMergedRegions = poiSheet . getNumMergedRegions ( ) ; mergedRegions = new ArrayList ( numMergedRegions ) ; for ( int i = 0 ; i < numMergedRegions ; i ++ ) { CellRangeAddress r = poiSheet . getMergedRegion ( i ) ; final RectangularRegion rr = new RectangularRegion ( r . getFirstRow ( ) , r . getLastRow ( ) , r . getFirstColumn ( ) , r . getLastColumn ( ) ) ; mergedRegions . add ( rr ) ; } } @ Override public int getNumberOfColumns ( ) { } @ Override public int getFirstColNum ( ) { } @ Override public int getLastColNum ( ) { } @ Override public int getFirstRowNum ( ) { } @ Override public int getLastRowNum ( ) { } @ Override public int getNumberOfRows ( ) { } @ Override public String getSheetName ( ) { } @ Override public abstract PoiWorkbook < WORKBOOK , SHEET , ROW , CELL , CELLSTYLE , COLOR > getWorkbook ( ) { } @ Override public float getColumnWidth ( int col ) { } @ Override public float getRowHeight ( int rowNum ) { } @ SuppressWarnings ( "rawtypes" ) @ Override public boolean equals ( Object obj ) { } @ Override public int hashCode ( ) { } @ Override public TableModel getTableModel ( ) { } @ Override public void freeze ( int i , int j ) { } @ Override public abstract PoiRow < WORKBOOK , SHEET , ROW , CELL , CELLSTYLE , COLOR > getRow ( int i ) { } @ Override public PoiCell < WORKBOOK , SHEET , ROW , CELL , CELLSTYLE , COLOR > getCell ( int i , int j ) { } @ Override public void autoSizeColumn ( int j ) { } @ Override public void setAutofilter ( int rowNumber ) { } @ Override public Iterator < Row > iterator ( ) { } static class PoiHssfSheet extends PoiSheet < HSSFWorkbook , HSSFSheet , HSSFRow , HSSFCell , HSSFCellStyle , HSSFColor > { private final PoiHssfWorkbook workbook ; private final PoiHssfCellStyle defaultCellStyle ; public PoiHssfSheet ( PoiHssfWorkbook workbook , HSSFSheet poiSheet ) { } @ Override public PoiHssfRow getRow ( int row ) { } @ Override public PoiHssfCellStyle getDefaultCellStyle ( ) { } @ Override public PoiHssfWorkbook getWorkbook ( ) { } } static class PoiXssfSheet extends PoiSheet < XSSFWorkbook , XSSFSheet , XSSFRow , XSSFCell , XSSFCellStyle , XSSFColor > { private final PoiXssfWorkbook workbook ; private final PoiXssfCellStyle defaultCellStyle ; public PoiXssfSheet ( PoiXssfWorkbook workbook , XSSFSheet poiSheet ) { } @ Override public PoiXssfRow getRow ( int row ) { } @ Override public PoiXssfCellStyle getDefaultCellStyle ( ) { } @ Override public PoiXssfWorkbook getWorkbook ( ) { } } }

Fix:
for ( int i = poiSheet . getFirstRowNum ( ) ; i < ( ( poiSheet . getLastRowNum ( ) ) + 1 ) ; i ++ ) {